The present invention relates to a method of manufacturing a curved element made of composite material.
As is known, some important curved structural elements, such as fuselage frames for example, can advantageously be made of composite material to improve their stiffness and reduce weight. These elements often require the carbon, fibreglass or other reinforcing fibres to have the same curved orientation as the structural element to improve its structural efficiency and reduce its weight.
This requirement cannot be satisfied with the use of conventional prepreg composite materials. In fact these are produced from groups of straight fibres, unidirectional fabrics, or strips which are impregnated with resin film in a continuous process. Owing to its continuous nature, the conventional pre-impregnation process is not suitable for the impregnation of reinforcing layers with fibres that are not straight.
Various methods have been developed to overcome the limitations imposed by pre-impregnation and to produce curved structural components but they have some disadvantages.
In particular, processes which partially solve the problem have been developed; these processes enable reinforcing layers with curved fibres, such as flat, curved, braided layers and spiral fabric layers to be assembled. However, these flat elements must be formed into the geometrical shape of the component to be produced (for example, C-sectioned frames or L-sectioned shear ties), prior to the infusion of resin. The shaping of these fabrics presents considerable difficulties since the non-impregnated fibres easily stray from the original orientation so as to be arranged in unpredictable ways inconsistent with the design. To overcome this problem, a technology based on a special weaving technique known as “overbraiding” has been developed. In this technique, the reinforcing fibre is woven, by means of special machines, onto a temporary support mandrel having the same curved shape as the component to be produced and having a cross-section such that the section of the component to be obtained can be produced therefrom simply by longitudinal cutting.
However, this innovative technology also has some important limitations. In fact:                it is not possible to orient the reinforcing fibres in all directions, as would be desirable to improve structural performance, but only within a more restricted range, normally within the range of from about +70° to −70° relative to the longitudinal axis of the element to be produced; it is therefore not possible to insert fibres arranged at 90° (relative to the longitudinal axis of the element), which are typical of the lay-up of composite structural components;        the delicacy of the preforms requires them to be supported by the auxiliary weaving mandrels from the time when they are woven until the time of use, creating problems with regard to costs and to transport and storage logistics; a corresponding number of expensive support mandrels is also required;        the cross-sections that can be produced by the overbraiding process are limited to those that can be obtained from a closed C-section; other cross-sections are obtained with difficulties that are accompanied by quality problems, that is: fibre orientation not to design, wrinkles, etc.,        the sections that can be produced by the overbraiding process are limited to a uniform thickness and cannot have variations relative to the transverse axis of the element,        the process for the infusion of the reinforcing preform with resin is particularly complex and limits the types of resin that can be used.        